This invention relates to an environmentally safe method for the disposal of incinerator ash. Specifically, this invention discloses a vitrification process whereby municipal waste combustor incinerator ash, either with a high or low halogen content, containing heavy metal species is subjected to high temperatures and vitrified into a homogeneous, single-phase glass.
Municipal solid waste incinerators produce ash which contains small quantities of heavy metals, such as lead, zinc, mercury, cadmium and chromium; generally less than 0.5% of each. The immobilization of these metals is necessary to prevent air and groundwater contamination in disposal or recycling. Conventionally, incinerator ash which contains heavy metal contaminants has been used as aggregate with Portland cement to form stable concrete structures. The leaching characteristics of the cured concretes do not consistently satisfy the Environmental Protection Agency's Toxic Characteristic Leaching Procedure (EPA's TCLP test) requirements for hazardous waste, thus allowing it to be delisted. Ash which is used as aggregate is also classified as nonhazardous by the particular states in which it is used. Nevertheless, in many cases, the leachability data is questionable, and the acceptable results may be a result of the dilution of the toxic components by the large volume of the filled concrete.
It is also known to convert waste material into glass by means of melting after adding additives and after mixing into batch. Some of the material is dissolved in the melt, i.e., it is chemically decomposed, and the rest is firmly incorporated in the glass when the latter solidifies after withdrawal from the furnace. It is advantageous that the glass be difficult to leach out, so that release of components contained in the glass can occur only to a small amount, if at all. This permits a disposal without further manufacture of bodies made from the glass. Additionally, vitrification results in a 20-80% reduction in volume of the waste material. This allows the life of a landfill to be expanded significantly.
However, there are still problems which often occur when vitrifying waste materials, especially if these waste materials contain a high percentage of chloride, sulfur and/or carbon species; those problems resulting from the fact that these species are usually absorbed by the melt only to a limited extent. For example, typical incinerator ash containing low chloride is usually combined with the high Cl scrubber residue, which is generated in state-of-the-art incinerators utilizing CaO injection scrubbers; the resultant ash-residue mixture then containing chloride levels greater than 15%. In an open melt operation the presence of these species typically leads to the disadvantageous formation of large amounts of exhaust gas which can contain Cl.sub.2, HCl, SO.sub.x, CO and the like. Furthermore, the heavy metal and alkali metal species are volatilized and they too enter the exhaust gas from the glass melt. All of this excessive effluent generation requires purification and extensive exhaust gas collection systems which involve significant cost. Although a cold crown melting system would alleviate the heavy metal volatilization, the excess chlorine still would cause excessive gaseous evolution, i.e., HCl and volatile metal chlorides. These, in turn, lead to batch bridging, formation of gas pockets and occasional formation of molten salt layers (e.g., CaCl.sub.2) in the batch blanket. Furthermore, any resultant glass may contain in excess of 10% Cl which reduces the ultimate acid durability below acceptable levels. In other words, high chloride incinerator ash cannot be conventionally melted, or even continuously melted in a cold crown melting system, without the removal of an appreciable quantity of the Cl.